Generally, gas turbine engines have three main sections or assemblies, including a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor assembly compresses ambient air. The compressed air is channeled into the combustor assembly where it is mixed with a fuel and ignites, creating a working combustion gas. The combustion gas is expanded through the turbine assembly. The turbine assembly generally includes a rotating assembly comprising a centrally located rotating shaft and a plurality of rows of rotating blades attached thereto. A plurality of stationary vane assemblies, each including a plurality of stationary vanes, are connected to a casing of the turbine assembly and are located interposed between the rows of rotating blades. The expansion of the combustion gas through the rows of rotating blades and stationary vanes in the turbine assembly results in a transfer of energy from the combustion gas to the rotating assembly, causing rotation of the shaft. The shaft further supports rotating compressor blades in the compressor assembly, such that a portion of the output power from the rotation of the shaft is used to rotate the compressor blades to provide compressed air to the combustor assembly.
A transition duct is typically used as a conduit for the passage of the combustion gas from the combustor assembly to the turbine assembly. The transition duct may be comprised, for example, of a forward cone section and an intermediate exit piece. The forward cone section may include a generally circular forward end that receives the combustion gas from a basket member of the combustor section. The forward cone section may converge into a generally circular aft end that is associated with a generally circular forward end of the intermediate exit piece. An aft end of the intermediate exit piece may include a generally rectangular shape and delivers the combustion gas to the turbine section.
Due to the high temperature of the combustion gas that flows through the transition duct, the transition duct is typically cooled during operation of the engine to reduce the temperatures of the materials forming the forward cone section and the intermediate exit piece. Such cooling is typically required, as the materials forming the forward cone section and the intermediate exit piece, if not cooled, may become overheated, which may cause undesirable consequences, such as deterioration of the transition duct.
Prior art solutions for cooling the transition duct include supplying a cooling fluid, such as air that is bled off from the compressor section, onto an outer surface of the transition duct to provide direct convection cooling to the transition duct. An impingement member or impingement sleeve may be provided about the outer surface of the transition duct, wherein the cooling fluid may flow through small holes formed in the impingement member before being introduced onto the outer surface of the transition duct. Other prior art solutions inject a small amount of cooling fluid along an inner surface of the transition duct. The small amount of cooling fluid acts as a cooling film to cool the inner surface of the transition duct. The cooling film is gradually heated up by the combustion gas, wherein the cooling film is mixed in with the combustion gas and is transferred into the turbine section along with the combustion gas.